During the drilling of a wellbore, various fluids are used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, a drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
Wellbore fluids may also be used to provide sufficient hydrostatic pressure in the well to prevent the influx and efflux of formation fluids and wellbore fluids, respectively. When the pore pressure (the pressure in the formation pore space provided by the formation fluids) exceeds the pressure in the open wellbore, the formation fluids tend to flow from the formation into the open wellbore. Therefore, the pressure in the open wellbore is often maintained at a higher pressure than the pore pressure. However, when wellbore pressures are maintained above the pore pressure, the pressure exerted by the wellbore fluids may exceed the fracture resistance of the formation and fractures and induced mud losses may occur. Further, the presence and/or creation of formation fractures may result in loss of wellbore fluid, which decreases the hydrostatic pressure in the wellbore, which may in turn also allow formation fluids to enter the wellbore. As a result, the formation fracture pressure may define an upper limit for allowable wellbore pressure in an open wellbore while the pore pressure defines a lower limit. Therefore, one constraint on well design and selection of drilling fluids is the balance between varying pore pressures and formation fracture pressures or fracture gradients through the depth of the well.
As stated above, wellbore fluids are circulated downhole to remove rock, as well as deliver agents to combat a variety of issues beyond the scope of the present disclosure. The selection of the type of a wellbore fluid to be used in a drilling application involves a careful balance of both the good and bad characteristics of the wellbore fluids in the particular application and the type of well to be drilled. Frequently, the selection of a fluid may depend on the type of formation through which the well is being drilled. Fluid compositions may be water- or oil-based and may contain weighting agents, surfactants, proppants, viscosifiers, and fluid loss additives. However, fluid loss may impede wellbore operations, as fluids escape into the surrounding formation. During drilling operations, variations in formation composition may lead to undesirable fluid loss events in which substantial amounts of wellbore fluid are lost to the formation through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole. While fluid loss is often associated with drilling applications, other fluids may experience fluid loss into the formation including wellbore fluids used in completions, drill-in operations, productions.
Lost circulation is an uncontrolled flow of a wellbore fluid (such as a drilling mud) into a fractured formation, and may occur naturally in formations that are fractured, highly permeable, porous, cavernous, vugular, or can be artificially induced by excessive mud pressures. Such openings in the formation may be naturally occurring or may be induced by the pressure exerted during the pumping operations. Lost circulation should not be confused with fluid loss, which is a filtration process wherein smaller amounts of the liquid phase of a drilling fluid or cement slurry escapes into the formation, leaving the solid components behind.
Lost circulation can be an expensive and time-consuming problem. During drilling, this loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returned fluid. Lost circulation may also pose a safety hazard, leading to well-control problems and the potential for environmental incidents. A particularly challenging situation arises in depleted reservoirs, in which the drop in pore pressure effectively weakens a wellbore through permeable, potentially hydrocarbon-bearing rock formation, but neighboring or inter-bedded low permeability rocks maintain their pore pressure. This can make the drilling of certain depleted zones impossible because the mud weight employed to support lower permeability rocks such as shale may exceed the fracture resistance of high permeability sands and silts. Another unintentional method by which lost circulation can result is through the inability to remove low and high gravity solids from fluids. Without being able to remove such solids, the fluid density can increase, thereby increasing the hole pressure, and if such hole pressure exceeds the formation fracture pressure, fractures and fluid loss can result. Thus, over the decades, lost circulation has been one of the most time consuming and cost inflating events in drilling operations.
Various methods have been used to restore circulation of a drilling fluid when a lost circulation event occurred, particularly the use of “lost circulation materials” (LCM) that seal or block further loss of circulation. LCM may generally be classified into several categories: surface plugging, interstitial bridging, and/or combinations thereof. In addition to traditional LCM pills, crosslinkable or absorbing polymers, and cement or gunk squeezes have also been employed to combat fluid loss downhole.